Various applications, including laboratory, industrial, and audio applications, for example, share the need to measure the relatively small voltage difference, referred to as a differential mode voltage, between two individual voltages when a potentially very large voltage, referred to as a common mode voltage, is common to both of the individual voltages. That is, the common mode voltage is the voltage shared by the inputs, where the differential mode voltage is the voltage not shared. Instrumentation amplifiers may fill this need by rejecting the superimposed common voltage, i.e., common mode voltage, on two individual input voltages, and generating an output voltage that is directly proportional to the difference between the two individual input voltages, i.e., differential mode voltage. Thus, the output voltage, VOUT, of an instrumentation amplifier is characterized by the operation of a common mode gain, ACM, and a differential gain, ADM, on the two input voltages, VIN1 and VIN2, according to Equation 1.
                              V          OUT                =                                            A              DM                        ⁡                          (                                                V                                      IN                    ⁢                                                                                  ⁢                    2                                                  -                                  V                                      IN                    ⁢                                                                                  ⁢                    1                                                              )                                +                                    A              CM                        ⁢                                          (                                                      V                                          IN                      ⁢                                                                                          ⁢                      2                                                        +                                      V                                          IN                      ⁢                                                                                          ⁢                      1                                                                      )                            2                                                          (        1        )            
An ideal instrumentation amplifier may have a constant differential mode gain (i.e. constant value for ADM) and may have a common mode gain (ACM) of zero, meaning the common mode voltage is completely removed from the output. In practice, the common mode gain (ACM) may not be zero but may be much less than the differential mode gain (ADM). An important measurement for instrumentation amplifiers is the common mode rejection ratio (CMRR), which is a ratio of the common mode gain (ACM) to the differential gain (ADM) expressed in decibels (dB). Typical instrumentation amplifiers may have CMRR in the range of 30 to 100 dB. The higher the CMRR, the closer an instrumentation amplifier is to ideal.
Typical instrumentation amplifiers are implemented using operational amplifiers and four or more resistors. The magnitude of resistance presented by particular resistors in the instrumentation amplifier may dictate the gain of the amplifier.